Rapid and reliable measurement of water quality is of major importance, particularly with respect to drinking water. Ultraviolet transmittance (UVT) is a measure of the transmittance of water to UV light. Fundamentally, this requires a UV light source to shine UV light through a water sample and into a UV detector. The UVT of a water sample is calculated as the amount of UV light that passes through the water sample under test (test sample) divided by the amount of light that passes through a water sample of known UVT (blank sample), preferably of 100% UVT. There are a number of existing products that make use of this method to measure UVT.
This method, often called the “single-beam” method, is well known in the art and has long been used in photometric instruments of various and multiple wavelengths. U.S. Pat. No. 4,832,491 describes the use of this method whereby the ratio described above is determined for each selected wavelength to enable the calculation of an absorbance spectrum of the sample.
One of the main difficulties when designing UVT instrumentation is due to the nature of UV light sources. The most common UV light source is the mercury lamp, which has a tendency to drift and fluctuate causing significant errors in the UVT measurements when using the single-beam method described above. Such fluctuation and drift is very common in UV lamps and is due primarily to changes in temperature and imperfections in the ballast and lamp.
Some low cost designs make no attempt to compensate for such fluctuations and drift which severely reduces accuracy. These designs require the user to wait up to 30 minutes after turning on the instrument to allow as much time for the lamp to stabilize as possible. However, significant errors are still common.
One way to reduce these errors is to use a feedback loop to allow the instrument to adjust the lamp output in an attempt to maintain a constant output. However, this is very costly to implement since additional electronics, a second sensor and a proportionally adjustable ballast is required to power the UV lamp.
Another way to attempt to reduce these errors is to use a second beam of light. This method is commonly called the “double-beam” method and is well known in the art and has long been used in photometric instruments of various and multiple wavelengths.
There are several different implementations of double-beam technology. One such implementation is to use a light source with two chambers and two light sensors in which one chamber is intended to accept a test sample and the other chamber is intended to accept a blank sample. A ratio of the output from each light sensor is used to determine the transmittance or absorbance of the sample under test. Since the first and second light sensors output the blank sample and test sample data at the same time, errors caused by lamp output drift and fluctuations are eliminated. However, this method introduces new errors due to the use of two sensors. Differences in the optics of each sensor location can produce non-linear differences between the measurements made using each sensor. Differences in the electronic signal path of each UV sensor can also significantly affect the measurements of each sensor. Also, if each sensor is looking at a different part of the lamp and/or looking at the lamp from a different angle, errors can occur since the UV lamp output varies not only over time, but also over the surface of the lamp. For these reasons, this particular design is not very common.
Another double-beam method uses only one light sensor so that the errors introduced with a second sensor are eliminated. A sample chamber and a reference chamber are still used, but instead of two light beams propagating through the two chambers to two sensors, one light beam is switched intermittently between the two chambers using a switchable beam splitting apparatus, where the light beam from one chamber at a time is incident on a single light sensor. U.S. Pat. No. 4,577,106 describes the use of this design using one light detector and a mirror capable of rotating with the purpose of directing the light beam through either sample or reference chambers at certain times.
However, there are still errors present in this method that are not present in the single-beam method. Since two sample vials are required, one for the test sample and one for the blank sample, significant errors can occur since the two sample vials are not necessarily matched. Also, this system is expensive due to use of a switchable beam splitting apparatus and the potential use of precision manufacturing to attempt to match the optics of the two beam paths. It should also be noted that the cost of such optical apparatus becomes especially expensive when using UV light since UV light does not readily transmit through glass lenses making the use a quartz optical components necessary. UV light also has a tendency to erode reflective surfaces, making the use of mirrors undesirable.
Yet another double-beam method is available. This method again uses two signal paths through two chambers. However, instead of inserting the water sample under test in one chamber and the reference sample in the second chamber, this method uses the first chamber for both the water sample under test and the reference sample, at alternate times, and the second chamber is left empty such that the light is allowed to pass freely to the light sensor to provide information on the amount of light output from the lamp at certain times. As before, the transmittance or absorbance is calculated using a ratio of the light transmitted through the test sample to the light transmitted through the blanksample, the same as the single-beam method and the other double beam methods.
However, the second light beam allows the light detector to determine the raw lamp output at the times when the light transmitted through the blank sample and the test sample were determined. This allows the instrument to compensate for lamp drift and fluctuations that occur over that time. U.S. Pat. No. 3,579,105 describes the use of this design using two light detectors. However, this method is still susceptible to errors caused by using the two separate light paths and light detectors, as discussed above.
Therefore, there is a need for a water quality measuring device which is compact, inexpensive and easy to use which avoids the aforementioned limitations. The present invention provides a device that measures the UVT/UVA of water samples while compensating for lamp drift and fluctuations, using only one light detector, one light beam, and without the need for expensive optical components such as UV resistant mirrors and/or quartz lenses.